Process of producing noncolloidal calcium carbonate



Patented Mar. 17, 1953 PROCESS OF PRODUCING NONCOLLOIDAL CALCIUMCARBONATE Milton 0. Schur, Asheville, and Robert M. Levy,

Brevard, N. C., assignors to Ecusta Paper Corporation, a corporation ofDelaware No Drawing. Application August 3, 1949, Serial No. 108,416

2 Claims.

This invention relates to the manufacture of calcium carbonate andespecially in a form suited for use as a filler in cigarette paper forimparting thereto the desired opacity, porosity, and thicknesscharacteristics.

In our copending application Serial No. 745,728, filed May 3, 1947, wehave disclosed and claimed a process of producing calcium carbonate incertain desired particle sizes for use in cigarette paper in connectionwith the porosity and opacity characteristics of the paper, the processcomprising a first step of producing calcium carbonate particle sizesfiner than desired for use in the paper and a second step of increasingthe particle size of these fine particles so as to obtain the optimumsize particles. Our two-step process is a decided improvement in thecontrol of particle size in the finished product over a singlestageprocess.

Nevertheless, there are characteristics of the calcium carbonate filler,other than particle size, that must be controlled to give the optimumproperties in the final high-quality cigarette paper, and while thesecharacteristics are difiicult to identify specifically, we believe thatthey include the varying tendencies of the particles to aggregate, theretention of the filler in the paper finish during the manufacturethereof, the softness of the filler, and the bulking properties of thefiller. These are apparently important characteristics, but we do notwish our invention to be limited to them or to any theory of why our newprocess produces the type of calcium carbonate filler that gives thedesired results in the finished cigarette paper.

In our research which led to the present invention, we found that one ofthe main problemswas to produce and reproduce consistently batches ofthe desired type of calcium carbonate. Despite strict control of theoverall process conditions, variations would occur in the quality of thefinal precipitated calcium carbonate, and the end result in any onebatch could not be definitely anticipated. We finally discovered thatthe carbonating conditions at the very start of carbonation areexceedingly important in determining the quality of the final filler,probably because the particles of calcium carbonate formed at the outsetconstitute nuclei, the physical-chemical characteristics of whichdetermine the general characteristics of the batch. In short, in theconsistent production and reproduction of the desired end product, themost important conditions during carbonation are those at the outset ofcarbonation. We discovered that the essential 2 condition is to have theslaked lime, calcium hydroxide suspension, initially at a temperature ofabout C. to 25 C. and preferably at about C. When this initial conditionhas been established and carbonating of the lime with CO2 begun, thetemperature of the mixture may be allowed to rise. Usually thetemperatures reach an equilibrium level of about 50 C.-60 C. Once theproper initial relatively low temperature has been established, it isnot necessary to maintain that temperature or any fixed low temperaturethroughout the carbonating reaction. This is contrary to prior beliefsof other workers in this field, as illustrated by the prior art patentsthat recommend maintaining the temperature during carbonation at a fixedlow value or within some specified narrow range of temperatures. Our newdiscovery in this regard is of substantial commercial value because itavoids the necessity of maintaining a low temperature during thecarbonating reaction, which is exothermic, and which would requireconsiderable cooling to absorb the heat of reaction.

We recognize that prior patents disclose processes for obtaining certaincharacteristics of calcium carbonate by control of the temperature ofcarbonation and other operating conditions. A typical prior art processof this type involves the maintaining during carbonation of a relativelylow temperature of the lime being carbonated, so as to produce acolloidal calcium carbonate. Our present invention fundamentallydistinguishes over these prior art processes in that we have found it isnot necessary to maintain the low temperature throughout the carbonationreaction, provided the initial tempera ture is adjusted properly and,also, by permitting.

the temperature to rise we avoid the production of colloidal calciumcarbonate. We have determined from tests that colloidalcalcium carbonateis much too fine in particle size to give the above-described desiredproperties in the cigarette paper and has other undesirable charac-vteristics such as drying to a hard, dense mass.

Pursuant to our invention, we definitely avoid the production ofcolloidal carbonate and in stead produce a non-colloidal calcium carbon?cooling of a thickened, i. e., concentrated, lime slurry with cold waterused for dilution to bring it to the proper concentration forcarbonating.

Regarding the carbon dioxide gas, which is introduced into the calciumhydroxide suspension for effecting carbonation, as described below, theCO2 concentration will vary with the type of gas used. For example, ifflue. gas is. used, it will contain approximately 8% to 12% CO2, whereasif kiln gas is used, the percentage will vary within the range of about10% to 30%; The percentage concentration of CO2 in the gas willdetermine the concentration of the. lime slurry to be used. For example,when flue gas is used, the lime concentration, calculated. as CaCOs,should be approximately 220 to- 280' grams per liter.

The process of our invention will be more clearly understood from thefollowing description of a typical laboratory preparation, and a typicalpilot plant production.

Laboratory preparation bonato'rv at the rateof 0.6'C. F. M. The 4."di.--

ameter turbo-mixer in the carbonator was re.- tated at about 500 R. P.M. (The equipment used is sold by the Turbo-Mixer Corporation as 4"laboratory turbo-gas absorber.)

The temperature of the batch was increased by means of aheating coil atthe uniform rate ofv 30 C. increase in temperature in the timerequiredto carbonateone-half of the lime present. This rate of increasein temperature corresponds approximately to the increasev obtained inlarge scale apparatus as abalance between theheat of reaction, on theonehand, andradiationand other heat losses, on the other hand.-

The: temperature was then held at 50 C. for

the remainder of the time of carbonation. Fromtime to time, asmallamount of water was added to: replace that lost by evaporation.About 12 hourswasrequired to convert all of thelime. initially present.to calcium. carbonate.

Handsheets. of cigarette paper made with numerous batches of. calcium.carbonate prepared according to the above procedure. and fromlimes. fromdifferent localities, consistently gave sheets.

having. opacity, porosity and thickness. values.- satisfactory incommercial made cigarette. papen.

Pilot plant production A. good commercial. grade of 1" high. calcium.quicklime burned in a. rotary was slaked. by the gradual. addition toboiling water. of. 2340: lbs. of limeto 880 gals..of water.. The limesus.-- pension was gently agitated with. compressed air for severalhours to insure complete hydrat'ion.

Of the above lime slurry, 7750 gallons were adjusted in concentration soas to contain 250 grams per liter expressed as CaCOs and were pumpedinto a 10,000-gal. commercial turbocarbonator. The lime slurry was thencooled with cooling water sprayed on the outside of the tank to about20" C. Boiler flue' gas containing;

about 10% CO2 was then added at the rate of about 800 C. F. M. under thesurface of the liquid near the blades of the rotating turbine. Theturbo-agitator was rotated at about 50 R. P. M. The temperaturegradually rose at a. relatively uniform rate until. alevel of about 50C. was attained. This corresponded approximately to the time when /2 ofthe lime initially present had been converted to calcium carbonate. Thetemperature then remained practically constant during remainder of thecarbonation period, the heat of reaction evidently being balanced byradiation and other heat losses. The time required to convert all thelime to carbonate was about 18 hours.

Batches of this pilot plant filler were first evaluated with the aid ofthe laboratory handsheets of cigarette paper. The results obtainedwereidentical with the corresponding filler similarly made in thelaboratory as described previously. In contrast with these results,similar batches of calcium carbonate made either in the laboratory or inthe pilot plant at a temperature initially above 30 C. gave variable andfrequently poor results, when tested in the cigarette paper.

Other methods than those described above for initially cooling the limeslurry prior to carbonation may be employed. They include the use of acontinuous centrifuge associated with a rotating discharge screw andoperating so as to produce from the hot lime slurry a relatively thick,.heavy mass of the hydrated lime, the. bulk of the initial water contentof the hydrated. lime: being thrown out. by the centrifuge. and. beingrecycled, if desired, for use in the slakingv of'another batch of lime.The discharge screw will slowly feed the dewatered thick. mass of.hydrated lime away from the centrifuge and into a suitable vessel fordilution of this heavy slurry with process. water at sufficiently lowtemperature to convert the heavy sludge or mass of slaked lime into awatery suspension having. a temperature of about 20 C. This procedure,which involves. first concentrating the lime slurry by centrifuging,and: then diluting the concentrated sludge thus produced with cold waterto effect a lowering of the temperature of the entire mass to 20 C.,has. a definite practical advantage from the heat transfer standpoint.It is difiicult to obtain quickly a lowering of the temperature of thecenter. portion of the mass by external cooling. This difficulty iseliminated, however, when cold water is introduced directly into theconcentrated mass. This water serves the dual function of reducing thetemperature of the resulting suspension to the desired low' point and ofbringing the lime slurry to the desired concentration.

While the process of our invention has been described hereinabove withparticular reference to use of the calcium carbonate, produced by thisprocess, in cigarette paper, in which it has a special value, it is tobe understood that. the process maybe utilized for producing carbonatesfor other end uses within the scope of our invention; In fact, any" casewhere it is desired to reproduce consistently a predetermined type ofprecipitated calcium carbonate, this process may be employed. Once thedesired particle size and other characteristics of the calcium carbonateare determined, this product can be reproduced much more consistently bymeans of the process of our present invention than by any other knownprocess; and as described above our process is particularlycharacterized by the use of the initial cooling of the lime slurry to atemperature within the approximate range of C. to 25 (3., and thenpermitting the temperature to rise during the carbonating procedure.

Our process is used to advantage whether or not the filler is made inone carbonating step. or by the two-step carbonating method de-I scribedin our copending application Serial No. 745,728, filed May 3, 1947. Ineither case, the method described herein possesses the importantadvantage of leading more consistently than heretofore to thereproduction of the final qualities desired in the calcium carbonate.

The scope of our invention is indicated in the appended claims.

We claim:

1. The process of controlling the characteristics of precipitatedcalcium carbonate so as to render it especially suitable for use as afiller in cigarette paper comprising first, slaking quicklime, nextcooling the slaked lime slurry to a temperature of approximately 15 C.to 25 C., then adjusting the concentration of the cooled slurry toapproximately 220 to 280 grams per liter, expressed as calciumcarbonate, introducing gas, containing approximately 8% to 12% carbondioxide, into this slurry to effect an exothermic carbonation reactionbetween the slurry and the carbon dioxide, continuing this exothermicreaction on a gradually increasing temperature scale to an equlibriumvalue of about 50 C. to 60 C. until substantially all of the lime hasbeen converted into calcium carbonate of a non-colloidal form.

2. The process of controlling the characteristics of precipitatedcalcium carbonate so as to render it especially suitable for use as afiller in cigarette paper comprising first, slaking quicklime, nextcooling the slaked lime slurry to a temperature of approximately 20 C.,then adjusting the concentration of cooled slurry to approximately 220to 280 grams per liter, expressed as calcium carbonate, introducing gas,containing approximately 8% to 12% carbon dioxide, into this slurry toefiect an exothermic carbonation reaction between the slurry and thecarbon dioxide, continuing this exorthermic reaction on a graduallyincreasing temperature scale to an equilibrium value of about C. to C.until substantially all of the lime has been converted into calciumcarbonate of a non-colloidal form.

MILTON O. SCHUR. ROBERT M. LEVY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,058,503 Rafton et al. Oct. 27,1936 2,081,112 Statham et al. May 18, 1937 2,140,375 Allen et al. Dec.13, 1938 2,386,389 Elkington et al. Oct. 9, 1945 2,442,525 Wrege et al.June 1, 1948 FOREIGN PATENTS Number Country Date 563,111 Great BritainJuly 31, 1944 OTHER REFERENCES Mellor, Comp. Treat. On Inorganic andTheor. Chem, vol. 3, p. 817, 1923, Longman. Greens and Co., N. Y.

1. THE PROCESS OF CONTROLLING THE CHARACTERISTICS OF PRECIPITATEDCALCIUM CARBONATE SO AS TO RENDER IT ESPECIALLY SUITABLE FOR USE AS AFILLER IN CIGARETTE PAPER COMPRISING FIRST, SLAKING QUICKLIME, NEXTCOOLING THE SLAKED LIME SLURRY TO A TEMPERATURE OF APPROXIMATELY 15* C.TO 25* C., THEN ADJUSTING THE CONCENTRATION OF THE COOLED SLURRY TOAPPROXIMATELY 220 TO 280 GRAMS PER LITER, EXPRESSED AS CALCIUMCARBONATE, INTRODUCING GAS, CONTAINING APPROXIMATELY 8% TO 12% CARBONDIOXIDE, INTO THIS SLURRY TO EFFECT AN EXOTHERMIC CARBONATION REACTIONBETWEEN THE SLURRY AND THE CARBON DIOXIDE, CONTINUING THIS EXOTHERMICREACTION ON A GRADUALLY INCREASING TEMPERTATURE SCALE TO AN EQUILIBRIUMVALUE OF ABOUT 50* C. TO 60* C. UNTIL SUBSTANTIALLY ALL OF THE LIME HASBEEN CONVERTED INTO CALCIUM CARBONATE OF A NON-COLLOIDAL FORM.